Diversity receiving method and apparatus for combining a plurality of rapidly changing signals

ABSTRACT

According to the receiving method for mobile reception has a plurality of individual receiving antennas, also known as diversity receiving method, on the individual antenna output signals an auxiliary modulation is superimposed and the antenna output signals are added to give a summation signal which as amplified selected intermediate-frequency summation signal is then demodulated in relation to amount and frequency or phase, the phase position of an individual signal in relation to the phase position of the summation signal and/or the amplitude contribution of an individual signal to the amplitude of the summation signal thereby being determined. This information is utilized for optimizing the useful signal with a view to optimum interference suppression for example in that the phases and/or amplitudes of the high-frequency individual signals are changed in dependence upon the phase position and/or amplitude contribution determined. With low circuitry expenditure this then gives a substantially improved reception quality in mobile systems. Receiving antenna systems for carrying out the method are described.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a receiving method comprising a plurality ofindividual receiving antennas in which a phase modulation issuperimposed on an antenna output signal, the antenna signals aresummated and the summation signal amplitude demodulated. The inventionalso relates to a receiving antenna system for carrying out the methodcomprising a plurality of individual receiving antennas, a phasemodulator, a summation circuit and an amplitude demodulator.

2. Description of the Related Technology

In mobile reception, for example reception of radio and/or televisiontransmissions in motor vehicles, reception disturbances occur whichconsiderably impair the reception. Such reception disturbances are dueto the incidence of the radio or television waves on the antenna frommore than one direction. This so called multipath reception occursbecause the radio or television waves do not only reach the antennadirectly from the transmitter but for example are reflected at buildingsand also reach the receiving antennas along other paths. The receptionpaths for the plurality of signals received by the receiving antennahave different lengths so that in the radio and television signal, inparticular with frequency-modulated carrier, interference disturbancesoccur, the resultant carrier thereby undergoing both an amplitudedemodulation and a phase demodulation. The latter then given theannoying reception disturbances which considerably impair the receptionand which due to the physical factors occur irrespective of the type ofantennas employed, whether they are telescopic antennas, electronicshort-rod antennas or electronic wind screen antennas.

In an article by R. Heidester & K. Vogt in NTZ 1958, No. 6, pages315-319, a receiving antenna system is for example described which forreducing this interference occurring due to multipath receptioncomprises a plurality of individual receiving antennas for mobilereception. Associated with each individual antenna in this knownarrangement is a receiver with which the amplitude of each individualsignal of the respective individual antenna is continuously determinedand monitored. The amplitudes determined are compared and the respectivestrongest signal of an individual antenna is used as reception signal.This type of diversity system, also referred to as parallel or receiverdiversity system, is however very extensive and complicated in circuittechnology because each antenna must be provided with a receiver. Inaddition, it is not certain that the individual antenna which furnishesthe strongest antenna signal and is connected to the radio receiver inaccordance with the aforementioned criterion necessarily furnishes thebest signal, this being true in particular of frequency-modulatedsignals.

EP 0 201 977 A2, DE 3,334,735 A2 and the journal "Funkschau", 1986,pages 42-45, disclose for example a further receiving antenna system ofthe type mentioned at the beginning in which switching from one antennato the other or from one linear combination of antenna voltages to otherlinear combinations is carried out when the reception quality dropsbelow a predetermined threshold. This method, also known as scanningdiversity or antenna selection diversity system, has however thesubstantial disadvantage that this switching operation is initiated onlywhen interference has occurred. To achieve a transition between theantennas or linear combinations of antenna voltages which issatisfactory for the user and cannot be heard by him the switching overmust take place extremely quickly and from the circuit technology pointof view this is difficult, very complicated and then nevertheless onlypossible to a restricted extent. A further substantial disadvantage ofthis receiving system also resides in that an antenna furnishing arelatively poor reception signal which is however just beneath theswitching threshold is kept in operation although other antennas furnishbetter reception signals with less interference. Furthermore, wheninterference occurs at the antenna which happens to be activatedswitching results to an arbitrarily selected following antenna or linearcombination of antenna voltages which may also be disturbed or, asdescribed above, lie just below the switching threshold. The receptionproperties of this diversity system are therefore not satisfactory.

DE 3,510,580 A1 discloses an antenna receiving method or system in whichthe phase of a reception signal is abruptly changed arbitrarily orstatistically and the resulting amplitude change is measured. Thesetting of the optimum phase position in this case is by the "trial anderror" principle in several steps. Thus, a phase change is initiated andit is determined whether this results in a better or worse totalamplitude. The result is stored in each case in a processor. Thus, the"trial and error" method can at times also result in a deterioration ofthe reception conditions.

U.S. Pat. No. 4,079,318 discloses a known receiving method comprising aplurality of individual receiving antennas in which a phase modulationis superimposed on an antenna output signal by means of a phasemodulator. In a summation circuit the phase-modulated output signal isadded to the non-phase-modulated other antenna output signal. Afterfrequency conversion and an intermediate frequency amplification in aconventional receiving circuit the summation signal is amplitudedemodulated in a following amplitude detector and a synchronousdetector. This gives a control signal with which a phase rotationelement is continuously regulated so that the input signals of thesummation circuit are brought to a uniform phase position.

this known method is intended for the transmission in the microwaverange for reducing the socalled fading effects, the microwavetransmission system receiving a single electromagnetic signal with twoor more antennas. This known receiving method is thus intended fordirectional radio links and thus for stationary antennas. With thisknown control method the receiving lobes of directional radio antennasare caused to follow up to obtain an optimum reception with stationaryreceiving antennas. The follow-up takes place in minutes or hours, i.e.is relatively slow. There is no necessity for providing highspeedfollow-up because the reception lobes, as stated, can change only slowlyin said periods of time. An application of this known method to mobileradio, in particular the VHF radio reception in motor vehicles with alowest transmitted frequency of 40 Hz, is thus not possible because inmobile reception follow-up times of less than 20 msec are necessary. Thefollow-up speed in mobile motor vehicle reception is thus several ordersof magnitude faster. Moreover, the considerably longer follow-up timespresent in the known method can lead to noise in the audible range aswill be explained below in detail regarding the prior art known from DE3,510,580 A1.

A highspeed automatic control as is necessary for the reasons given inmobile radio diversity systems is not only unnecessary in stationarydiversity systems but is impossible therein because of the interferencewhich occurs. In the method known from U.S. Pat. No. 4,079,318 the oneantenna output signal is phase-modulated with a low-frequency signal.The control must therefore necessarily take place slowly because withthis low-frequency modulation the fluctuations of the interferencequantities can be followed only with a relatively low speed. For thisreason as well this known stationary diversity receiving method is notsuitable for mobile radio diversity reception.

Furthermore, in the known method the frequency bandwidth for thebaseband signal is also limited and the bottom frequency range cannot beutilized because of the low-frequency modulation signal. For this reasonas well the known receiving method is not suitable for mobile radiodiversity reception.

In the stationary diversity receiving systems in the microwave range thephase deviations due to the control operation then starting arerelatively small and do not exceed 90° phase deviation. As a result thecontrol signals also only lie within a narrow signal or voltage range.The control signal always remains proportional to the phase deviation.The drop of the control signal occurring at very large phase deviationsand the indifferent range occurring at 180° phase deviation are notreached. In contrast, in mobile diversity systems large phase diviationsand jumps extending over the entire phase range of 360° and high changerates (Rayleigh fading) are to be expected. In this case a controlmethod as described in EP-02 27 015 A2 for stationary diversity systemscould not be used because of the slow response and also because theregulation would become even slower in the event of indifferences.

A further basic difference between diversity receiving methods forstationary directional radio transmissions and mobile reception in motorvehicles resides in particular also in that the individual antennas indirectional radio systems in contrast to mobile radio diversity antennasfor example in motor vehicles always receive a signal whereas in mobileradio due to the very different incidence angle and characteristics eachantenna can receive the signal in any phase and amplitude position up tocomplete absence. Thus, whereas stationary antennas for opticalalignment with the receiving lobe only need to follow up relativelyslowly, in the case of mobile diversity systems it is necessary not onlyto detect and process phase changes but also amplitude changes inextremely wide change ranges. This is not possible with the receivingmethod known from EP 02 27 015 A2.

SUMMARY OF THE INVENTION

The invention is therefore based ont he problem of providing a receivingmethod and a receiving antenna system which is suitable for mobileradio, in particular for motor vehicle radio reception, and is able todetect and process very fast changes of the signals received by theindividual antennas, for example VHF signals, both as regards phase andamplitude over wide change ranges, and can utilize the information thusrecovered for optimizing the diversity reception.

Proceeding from the receiving method known from U.S. Pat. No. 4,079,318this problem is solved according to the invention in that on theindividual antenna output signals an auxiliary modulation issuperimposed in the form of a phase and/or amplitude modulation by meansof an auxiliary modulation signal, the summation signal amplified andselected in a receiving circuit is demodulated in a frequency andamplitude demodulator in relation to magnitude and frequency and/orphase, the auxiliary modulation signal is filtered out of thedemodulated signal and with the aid of synchronous demodulators real andimaginary parts of the individual antenna signal are determined inrelation to the summation signal and therefrom the phase position andamplitude contribution of the individual signal in relation to thesummation signal are derived, and the phases and/or the amplitudes ofthe high-frequency individual signals are each changed in dependenceupon the determined phase position and/or the determined amplitudecontribution in the direction towards optimum amplitude contribution.

On the basis of the steps according to the invention of superimposing anauxiliary modulation on the individual antenna voltages, subsequentlyadding all the antenna voltages and supplying the resultant summationsignal to the receiving circuit, it is possible by evaluating theresultant modulation of the summation signal on demodulation thereof todetermine the phase position of the individual antenna signals inrelation to the phase position of the summation signal and/or theamplitude contribution of the individual signals to the amplitude of thesummation signal. With the information obtained in this mannersubsequent controls or switching operations can be carried out foroptimizing the individual signals or the signal further processed int hereceiver.

Due to the auxiliary modulation it suffices in the present invention toprovide only one receiver. Nevertheless, information on all the antennavoltages is available so that said information can be used at any timefor optimizing, controls and switching operations.

A further advantage of the receiving method according to the inventionresides in particular also in that it reacts more or less continuouslyto deviations from the optimum and not only when interference hasalready occurred. In this manner substantially more time is availablefor the optimizing and for a control or switching operation than wouldbe the case if it were necessary to wait for occurrence of interference.

Another advantage is that the receiving system is always aligned withthe signal with the least interference irrespective of the startingconditions. This means that the receiving system does not stay with acertain receiving signal or a linear combination of receiving signalswhich lie or lies just beneath a switching threshold as is the case withconventional methods. On the contrary, continuous setting to a betterantenna or a better linear combination of individual signals is effectedand as a result the reception quality can be considerably increased.

Since in the receiving method according to the invention an in-phasesummation of the individual antenna signals takes place signal energiesof all the individual antennas can be utilized because the sum of allthe antenna signals is thereby better than each individual signal and inparticular interference of an individual antenna is statisticallyaveraged out so that from this point of view as well a considerablyimproved reception quality is obtained. With the receiving methodaccording to the invention it is also possible in the case offrequency-selective interference to generate a combined undisturbedreception signal from a plurality of disturbed individual signals. Alsoadvantageous is that the receiving method according to the inventionoperates independently of selective attenuation by shading in travellingoperation, also known as fading.

The auxiliary modulation of the individual antenna output signals mustbe evaluated for each signal in each case independently of the othersignals. Thus, the modulation of the individual antenna output signalscan be made in time succession. It is however also possible to carry outthe auxiliary modulation for the individual antenna output signals withdifferent frequencies, and the modulation can then possibly be madesimultaneously.

Thus, in the diversity receiving method according to the invention thesummation signal is thus demodulated not only in amplitude but also inquantity and phase in relation to the summation signal so that asregards both the phase position and the amplitude contribution aregulation or control is possible. Moreover, both the real and theimaginary part of the respective individual antenna signal is determinedin relation to the summation signal so that apart from the amplitude andphase the sign is also obtained. Consequently, not only phase rotationsbut also jumps to opposite phase as involved in the zero passagestypical for Rayleigh fading can be reliably detected and compensated ina single switching operation.

In contrast to aforementioned DE 3,510,580 A1 in the present methodaccording to the invention both the phase and the amplitude of theindividual signals in relation to the summation signal are uniquelymeasured and in dependence thereon a specific exactly defined setting ofthe phase elements made. The number of working steps is therebyconsiderably reduced. This has the advantage that correspondingly lessswitchovers are necessary and thus residual interference generated byswitchovers is also substantially reduced in the present invention. Forin order in the known arrangement to react fast enough before reaching aminimum, in the case of a fast automobile travelling for example at 150kilometers per hours, when the time interval of two consecutivereception minima in the VHF range in the most favourable case is 1.5metre/150 km/h, i.e. 36 milliseconds, the time of only 36milliseconds/4=9 milliseconds per antenna thus must not be exceeded ifthe diversity system has four antennas. To avoid a minimum at least fourmeasurements are necessary. This means that the measuring time is of theorder of magnitude of 2.25 milliseconds, i.e. lies in a frequency of 440Hertz. This frequency is however precisely in the range of maximumsensitivity of the ear so that the noise due to frequent switchover inthe known circuit is considerable and any reception improvement possiblyobtained are offset by this noise due to switchover. Since the auxiliaryfrequency lies in a measuring channel which is not used the usefulsignal is not influenced at all by the measurement.

The fundamental difference between the circuit arrangement known from DE3,510,580 A1 and the present method resides in particular also in thatthe antenna signals are converted in mixers used therein to a differentfrequency. In contrast, the antenna signals int he present inventionremain in the original frequency position, additional components onlybeing added thereto. This means that with the adder used in the knownmethod the summation is carried out in the intermediate-frequency rangeand not in the high-frequency range as is the case with the presentinvention. By definition, the following intermediate-frequency amplifierof the known method lacks the components necessary for frequency settingfor a receiving circuit, for example r.f. circuit and mixer. These partsmust be present separately for each antenna and this represents aconsiderable expenditure on circuitry. In contrast thereto, in thepresent invention the signals are added on the high-frequency sidebefore the receiving circuit so that usual radio receivers with onlyvery minor adaptations to the diversity system may be employed.

Moreover, the amplified and selected summation signal in the circuitarrangement according to DE 3,510,580 A1 is demodulated by the amplitudedemodulator as total signal. In the present invention the amplified andselected summation signal is demodulated in relation to the magnitudeand/or phase of the auxiliary modulation components. In contrast to theknown circuit arrangement the phase information is only then slightlyinfluenced by the interference amplitude modulation occurring inmultipath reception. As a result information is obtained ofsubstantially higher quality.

The diversity system described in EP 02 27 015 A2 is equipped with tworeceiving antennas. When using more than two receiving antennas all thedifferential phases must be measured. This requires n(n-1) measuringarrangements, i.e. a considerable apparatus expenditure. In contrast, inthe present invention the phase separation of the individual signals inrelation to the summation signal is made in a direction which is alwaysclear and points to the optimum. The apparatus expenditure is limited toa number of amplitude modulators and phase rotation elements increasinglinearly with the antenna number. The evaluating means itself is alwayspresent only once.

Preferably, the signal demodulated in relation to magnitude andfrequency or phase is demodulated coherently with the auxiliarymodulation signal. A selection of the auxiliary signal from thedemodulated signal mixture is obtained. The thereby resulting in-phasecomponents of the auxiliary modulation represent real and imaginaryparts of the phase-shifted auxiliary modulation signal. If for examplethe auxiliary modulation is an amplitude modulation the synchronouslydemodulated output signal of the amplitude demodulation detectorfurnishes the real part. The frequency modulation demodulator furnishesthe differentiated phase modulation signal. From this, with the aid ofthe synchronous demodulator the imaginary part shifted through 90° inphase is obtained.

If the synchronous demodulators in contrast to the previously describedcase are operated with the auxiliary modulation signal phase-shiftedthrough 90°, in a circuit arrangement with ideal components, i.e.components which for example do not effect any distortions or otherinterferences of the signal to be processed, the output signal mustdisappear. Normally, the radio sets for use with the receiving methodaccording to the invention are predetermined or exist. However, suchradio sets are made up with components which are not ideal components.Consequently, the ideal state described above where the output signaldisappears cannot be achieved. According to a further development of theinvention the demodulated signal is therefore additionally coherentlydemodulated with the auxiliary modulation signal phase-shifted through90° and the signal obtained in this manner used to compensateinterference arising through non-ideal components necessary forimplementation of the receiving method.

It is particularly advantageous in accordance with a further developmentof the invention when the auxiliary modulation is an amplitude and phasemodulation for the ratio between the two modulation types to becontrolled by the useful signal. In this manner disturbing influencesdepending ont he instantaneous signal state can be compensated.

A further embodiment of the invention resides in that the phases and/orthe amplitudes of the high-frequency individual signals are changed independence upon the phase position determined and/or the amplitudecontribution determined only in predetermined phase and/or amplitudestates.

This means that certain preferred and in particular expedient states arepredefined which are selected and set or activated correspondinglydepending on the phase position determined and/or the amplitudecontribution determined. In this manner it is also ensured in particularthat only expedient and clear states are selected and set.

It is particularly advantageous in this connection to change the phasesof the high-frequency individual signals in such a manner that onlyunilateral directional patterns are generated. This makes a multipathreception still more improbable.

As auxiliary modulation both a phase and an amplitude modulation ispossible as well as a combination of these two modulation types.

A particularly preferred embodiment of the receiving method according tothe invention resides in that the particular evaluated high-frequencyindividual signal is rotated in the direction of the phase of thesummation signal. As will be described in detail below with reference tothe example of embodiment, in this manner a continuous optimizing of theinput signal for the receiving circuit is achieved in the sense thatinterference due to multipath reception is minimized particularly well.In this embodiment of the receiving method the respective high-frequencyindividual signal is rotated in the direction of the phase of thehigh-frequency summation signal, i.e. the phase of the high-frequencyindividual signal is controlled and varied in dependence upon the signalcontaining information on the phase position and/or the amplitudecontribution of an individual signal in relation to the phase positionor amplitude of the summation signal in such a manner that the phase ofthe high-frequency individual signal is aligned with the phase of thesummation voltage. In this manner optimum interference suppression isobtained for the reception signal to be evaluated in the receiver.

On changing the phase of the respective high-frequency individual signalit is advantageous for the antenna patterns to be chosen so that theyare substantially circular patterns which have a pattern form which isas uniform as possible. In this manner optimal directional patterns canbe synthesized.

It is advantageous is the auxiliary modulation is consecutivelysuperimposed on the individual antenna output signals in a predeterminedclock sequence. The clock sequence should preferably be of the order ofmagnitude of milliseconds so that in the VHF range, i.e. in the 100 MHzrange, with a vehicle speed of 150 km/hour a plurality of samplings perquarter-wavelength travelling distance is made.

An alternative embodiment of the invention resides in that the clocksequence is controlled by the speed of a vehicle, the signal forcontrolling the clock sequence preferably being derived from thetachometer of the vehicle. By controlling the clock sequence independence upon the vehicle speed the modulation clock sequence of theindividual signals can be still further optimized.

A further development of the receiving method according to the inventionresides in that the starting instant at which auxiliary modulations aresuperimposed preferably consecutively ont he individual antenna outputsignals is controlled by the useful signal itself. This is particularlyadvantageous when the clock sequence is triggered on occurrence of thezero passage of the useful signal. In this manner it is ensured that thecorresponding measurement of the phase position of an individual signalin relation to the phase position of the summation signal and/or of theamplitude contribution of an individual signal to the amplitude of thesummation signal is always carried out at the same frequency in thetransmission channel. As a result, no measurement errors can arise dueto different frequencies or amplitudes of the useful signal.

A particularly advantageous further development of the receiving methodaccording to the invention resides in that from the antenna outputsignals linear combinations are formed on which the auxiliary modulationis superimposed. This means that the auxiliary modulation is notsuperimposed on the antenna output signals but on linear combinationsthereof. The use of linear combinations of the antenna output signalsmakes it possible to improve still further the interference suppressionin receiving systems.

As in the example of embodiment already explained according to which theindividual antenna patterns are at least approximately circular patternsit is also advantageous here for the same reasons if the antennapatterns formed by linear combination are at least approximatelycircular patterns.

In particular, a further development of the method according to theinvention is very advantageous in which the modulation parameters of themodulated antenna output signals are selected in such a manner that theauxiliary modulation frequencies occur in an unused frequency range of atransmission channel. This thus applies also to the modulated summationsignal. It is ensured in this manner that the signals themselves are notdisturbed by the modulations according to the method described here.

The invention can be applied advantageously to the reception of stereomultiplex signals. In this case the spectral components added by theauxiliary modulation to the antenna signal or linear combinations shouldlie in a frequency range outside the useful range, i.e. above 57 kHzand/or around 17 and/or 21 kHz.

A further development of the invention resides in that as control orswitching signal, for instance for rotating the particular evaluatedhigh-frequency individual signal in the direction of the phase of thesummation signal, a digital signal is employed. For this purpose,according to the invention the phase difference between the respectiveindividual signal and the summation signal and/or the amplitudecontribution of an individual signal to the amplitude of the summationsignal is converted to a digital signal.

The quantization of the phase may be chosen as desired but preferably a2-bit or a 3-bit quantization of the phase is selected, corresponding toa ±90° or a ±45° angle. The analog-digital conversion can thereby belimited to a simple threshold detection so that the circuit arrangementis still further simplified. A quantization with higher bit number doesnot in practice provide any appreciable improvement of the receptionquality. Even a 1-bit quantization with a ±180° switching angle alreadygives an appreciable improvement in the reception quality.

According to a further advantageous embodiment of the invention anindividual receiving antenna is deactivated when the amplitudecontribution of an individual signal to the summation signal drops belowa predetermined threshold value. This is made possible by information onthe amplitude of the individual signals being present so that independence upon this information for example in the manner describedabove individual antennas are deactivated or other switching stepsadditionally executed. The use of this information on the amplitude fordeactivating the individual antennas with small reception signalcomponent is however particularly advantageous because such antennaswith small amplitude component make substantially only a noisecontribution to the overall system and are thus in effectdisadvantageous. The reception signal of such a disconnecting antennacan nevertheless still be continuously measured if the carrierlesstwo-sideband-modulated signal of the antenna is supplied to thesummation signal.

A further preferred embodiment of the invention resides in that theauxiliary modulation is superimposed on the respective individualsignals during a predetermined time interval per period of the usefulsignal. It is ensured in this manner that the superimposing of theauxiliary modulation on the respective individual signal is made at adefined point of time, preferably when at said fixed point of time apredetermined constant amplitude is present. This embodiment isparticularly advantageous when the useful signal is a video signal andthe auxiliary modulation is superimposed during the line or frameblanking interval of the respective individual signal. The line or frameblanking interval has substantially the same amplitude in each period.As a result constant defined conditions are given and no measuringerrors of the system can arise due to different levels as would be thecase if the auxiliary modulation were superimposed during the intervalin which the actual video information is transmitted with differentamplitudes. Through this further feature the modulation of theindividual signal is subjected to a time condition so that within theperiod a sort of time-multiplex method results.

A further embodiment of the invention also provides that the receivingmethod is deactivated or activated entirely or partially in dependenceupon a control signal. Of course, relatively strong interference signalsoccur when electrical auxiliary motors for example for rearview mirroradjustment or windscreen wipers are switched on. In dependence thereon,individual antennas can be deactivated or other steps taken forinfluencing the overall system.

According to a further development it is also possible to activate thereceiving method only when a predetermined interference threshold isexceeded. For example, for determining whether the interference levelhas exceeded a predetermined threshold value an interference detectormay be provided.

The transmission behaviour of the receiving circuit in practice is notideal as regards the phase and/or amplitude profile so that errors canoccur in the determination of the phase position and the amplitudecontribution of the individual signals in relation to the phase andamplitude of the summation or composite signal. To compensate theseerrors it is advantageous if in accordance with a further embodiment ofthe invention on the summation signal for calibration a furtherpredetermined defined auxiliary modulation is superimposed by means of acalibration signal. Thus, with this additional feature with the aid of acalibration signal calibration of the receiving circuit is performed sothat errors of the receiving circuit which may negatively influence thetransmission of the signals can be compensated. The calibration signalcan be processed in a manner analogous to that provided for theauxiliary modulation signal for the individual antenna output signals sothat the aforementioned embodiment can also be applied to thecalibration signal.

In conjunction with the latter embodiment it is advantageous for thefurther auxiliary modulation to be identical to the auxiliary modulationwhich is superimposed on the individual antenna output signals.

According to a further development of the invention the furtherauxiliary modulation used for the calibration is superimposed on asummation signal which is formed from a subset of the antenna outputsignals and which presumably comes closest to the summation signalresulting after the change of the phase and/or amplitude of the measuredindividual signal. With this embodiment a faster approximation to theoptimum summation signal obtainable is possible because the individualsignal subjected to the change is not contained in the subset of theantenna output signals used to form the reference signal. If for exampleindividual antenna output signals have been found to be particularlydisturbing with the aid of an interference detector, these signals canalso be excluded from the formation of the reference signal.

In conjunction with the previously described features of the inventionit is furthermore advantageous if on the individual antenna outputsignals except for the antenna output signal to be changed an auxiliarymodulation is imposed in the form of a phase and/or amplitudedemodulation by means of an auxiliary modulation signal and that at thesame time the auxiliary modulation is superimposed on the antenna outputsignal to be changed with opposite sign. It is possible with thisembodiment to double the effective modulation depth of the antennaoutput signal to be changed compared with the modulation depth achievedwith exclusive modulation of a single antenna signal. This makes itpossible to achieve a high signal-noise ratio for the same modulationdistortions.

A further advantageous development of the invention resides in that themodulation of the summation signal is effected by simulataneousmodulation of all the antenna output signals. This is an alternative tothe embodiment in which on the summation signal for calibration afurther predetermined defined auxiliary modulation is superimposed bymeans of a calibration signal.

In the latter case, i.e. when on the summation signal for calibration afurther predetermined defined auxiliary modulation is superimposed bymeans of a calibration signal, it is advantageous to provide a modulatorbetween the summator and receiving circuit.

The problem set is solved according to the invention also with areceiving antenna system which has the following features:

A modulator which follows the individual receiving antennas and whichsuperimposes on the individual antenna output signals an auxiliarymodulation by means of an auxiliary modulation signal (S_(H)), ademodulator which demodulates the summation signal amplified andselected in a receiving circuit in relation to quantity and/or frequencyand/or phase, a filter which filters the auxiliary modulation signalfrom the demodulated signal, a synchronous demodulator which determinesthe real and imaginary parts of the individual antenna signal inrelation to the summation signal and derives therefrom the phaseposition and amplitude contribution and a phase and/or amplitude settingmember which is or are controlled in dependence upon the output signalsof the synchronous demodulators.

For a particularly economical version the demodulator can consist onlyof a frequency or amplitude demodulator. For this, the demodulator inthe radio set may be employed.

It is advantageous for a synchronous demodulator to be connected afterthe demodulator for coherent demodulation with the auxiliary modulationsignal. As already described, in this manner it is possible to make aselection by real and imaginary parts and thus determine the sign and asa result the phase position and the amplitude contribution of therespective individual signal in relation to the summation signal can beclearly determined.

According to a particularly advantageous embodiment of the inventionbetween the respective modulator and the summation circuit a phaseand/or amplitude control element is provided. The output signal of thedemodulator is supplied to the phase control element so that theparticular evaluated high-frequency individual signal can be rotated inthe direction of the phase of the summation signal. This gives acontinuous fast optimizing of the useful signal with regard tointerference reduction.

It is particularly advantageous to provide between the demodulator andthe respective phase shift element an analog/digital converter. In thismanner the control of the respective phase shift element is madedigitally in dependence upon the output signal of the demodulator andthis leads to simplification of the circuit arrangement.

Preferably, the modulators connected to the antenna outputs areactivated consecutively by means of a clock generator. Said clockgenerator may be fixed as regards its clock frequency or alternativelycontrolled by the demodulator output signal or otherwise controlled.

According to an alternative configuration of the invention it ispossible instead of in time succession to carry out the measurement foreach antenna output signal or each linear combination of antenna outputsignals, using for each signal a different frequency, auxiliaryfrequency generators and synchronous demodulators then accordinglyhaving to be multiply present.

A further development of the invention resides in that the modulatorsare preceded by a matrix circuit which forms linear combinations fromthe antenna output signals. By linear combinations of the actual antennaoutput signals a decorrelation and norming of the combined signals canbe achieved and thus a further improvement of the reception propertiesand interference suppression of the receiving antenna system accordingto the invention.

The demodulator advantageously includes an amplitude demodulator. It isparticularly advantageous for the amplitude demodulator to be a quasisynchronous demodulator. In this manner a demodulation is effected bymixing with the recovered signal carrier of the reception signalamplitude-modulated with the auxiliary modulation. The signal carrier isrecovered by limiting the reception signal. As a result, in the case ofcommon-channel interference for example in the FM radio reception rangeonly the quantity of the reception signal stronger at that instant ismeasured. As a result, on changing from one transmitter to the other thesummation signal can become stronger before the modulation thereof istaken on.

The invention will be explained hereinafter with the aid of an exampleof embodiment for sound ratio reception.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in schematic illustration a circuit arrangement to explainthe receiving antenna method or system according to the invention,

FIG. 2 shows a diagram of the signals in vector representation toexplain the mode of operation of the method and system according to theinvention, and

FIG. 3 is a schematic block diagram illustrating a preferred embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Via individual antennas 1-1, 1-2, 1-3, 1-4 the antenna output signalsare applied to a matrix circuit 2 at the outputs of which respectivelinear combinations of the antenna input signals are made available.Such matrix circuits are generally known and described for example in EPO 201 977 A2 so that they need not be discussed in detail here. Theoutputs of the matrix circuit 2 are each connected to an input of anamplitude modulator 3. As will be further described in detail below, anauxiliary modulation is consecutively superimposed on the signalspresent at the inputs of the amplitude modulator 3, the time clocksequence being controlled via the clock signal inputs in such a mannerthat depending on the clock signal input to which the clock signal isapplied the corresponding input signal occurs amplitude-modulated at theassociated output of the amplitude modulator 3. The amplitude modulator3 shown schematically as an array consists of four separate amplitudemodulator stages which each receive one of the output signals of thematrix circuit. The individual separate amplitude modulator stages areactivated in time succession in dependence upon the clock signal andcorrespondingly furnish in time succession the correspondinglyamplitude-modulated high-frequency input signals.

The amplitude modulator 3 is followed by a phase-shift element 4 whichfor the respective amplitude-modulated high-frequency input signals viaclock signal inputs from the same clock signal which is also applied tothe amplitude modulator 3 effects the phase rotation consecutivelyoccurring for the input signals. As will be explained in detail below asignal controlling the phase rotation is supplied to the phase-shiftelement 4. The phase-shift element 4 consists of four separatephase-shift elements which are each associated with a respective outputof the amplitude modulator 3 and are consecutively activatedcorresponding to said clock signal. The output signals of thephase-shift element 4 are added in a summation circuit 5 and supplied tothe input of a radio receiver 6 comprising a receiving circuit 7. In thecase of stereo reception the output signals R and L pass viacorresponding lines to the respective loudspeakers.

An intermediate-frequency output signal of the receiving circuit 7withdrawn from said circuit prior to the limiter stage passes via anamplifier and filter stage 8 to an amplitude and frequency demodulator 9and 10 respectively which are each followed by a synchronous demodulator11 and 12 respectively. Said demodulators 9, 10, 11 and 12 are circuitswith which the expert is familiar. The output signals of the synchronousdemodulators 11 and 12 following the amplitude and frequencydemodulators 9 and 10 pass to an analog/digital converter 13, the outputof which is connected to the control input of the phase-shift element 4.

A clock signal generator 14 in the form of an oscillator circuitgenerates the already mentioned clock signal S_(T) which for successiveactivation of the amplitude modulator 3 and the phase-element 4 isapplied to the clock signal inputs of said circuits. The clock signalgenerator 14 is controllable as regards its starting instant with theoutput signal of the frequency demodulator 10 via a trigger circuit 15.

An auxiliary modulation signal generator 16 provides the respectiveclocked and thus activated input of the amplitude modulator 3 with theauxiliary modulation signal S_(H) for consecutive modulation of therespective high-frequency individual signal. Said auxiliary modulationfrequency individual signal. Said auxiliary modulation signal S_(H) alsopasses to the synchronous demodulator 11 following the amplitudedemodulator 9 and with a phase shift of 90°, effected in a phase shifter17, to the synchronous demodulator 12 which follows the frequencydemodulator 10.

The mode of operation of the example of embodiment illustratedschematically will be explained below with reference to FIG. 2.

The auxiliary modulation S_(H) is superimposed on the output signals ofthe antennas 1-1, 1-2, 1-3, 1-4 or the linear combinations of theantenna output signals generated in the matrix circuit 2 correspondingto the clock sequence defined by the clock signal. Preferably, themodulation parameters are chosen so that the resultant spectrum lies inan unused range of the stereo multiplex signal, for example at 62 kHz.The output signals of the amplitude modulator 3, i.e. the modulatedindividual signals, pass via a phase-shift element 4 in each case to thesummation circuit 5 which forms from the individual signals a summationsignal which, as already mentioned, is taken as intermediate-frequencysignal from the receiving circuit 7 and amplified and filtered int heamplifier and filter stage 8.

In the amplitude and frequency demodulator 9 and 10 the signal mixtureis demodulated by the two detectors in relation to magnitude and phase.Thereafter, by the synchronous demodulators 11 and 12 real and imaginaryparts of the auxiliary modulation signals and their sign are determined.The output signals of the synchronous demodulators 11 and 12 contain asreal and imaginary parts the information on the amplitude contributionof an individual signal to the amplitude of the summation signal and thephase position of an individual signal in relation to the phase positionof the summation signal. These signals are quantized in theanalog/digital converter 13 for example with two bits and pass to thephase-shift element 4 effecting the corresponding phase rotation.

As stated, the output signals of the synchronous modulators 11 and 12contain the information relating to the phase position of the individualsignal relatively to the phase position of the summation signal andaccordingly the phase-shift element is controlled in such a manner thatin dependence upon the instantaneously present phase position and theinstantaneously present amplitude contribution to the summation signalfor the measured individual signal it alters the phase thereof in themanner which will be explained below with the aid of FIG. 2.

In FIG. 2 the phase positions of the individual signals are representedby full-line vectors V₁, V₂, V₃, V₄ which form a sum vector V_(S) shownin dash line. Now, in the output signal of the synchronous demodulators11 and 12 for example for the individual signal V the information on themagnitude of the phase difference of said signal from the summationsignal V is contained. Accordingly, this signal digitized in theanalog/digital converter 13 causes the phase-shift element 4 for thepresent modulated input signal to effect a phase shift through saidphase difference, in the present case of quantization with 2 bitsthrough 90°, and thus performs an alignment of the phase angle of thevector of the individual signal V₁ with the summation signal V_(S). Theindividual signal V₁ shifted in phase is represented by the vector V'₁.

With a finer quantization of the phase the remaining vectors could beadapted in corresponding manner to the phase of the summation signalS_(S). Corresponding to steps of 90°, however, as described in thepresent case only the individual signal V₁ is rotated in phase becauseit differs by more than 45° from the phase angle of the summation signalV_(S). However, quantization of the phases with 2 bits suffices in mostcases as is apparent from FIG. 2 with regard to the new summation signalV_(S), with the shifted individual signal V₁.

In FIG. 2 at the end of the arrow of the individual signal vector V₁ thevectors for the auxiliary amplitude modulation V_(S) are shown. In thepresent case the phase of the individual signal vector V₁ is turnedthrough about 90° in relation to the summation signal V_(S). As a resultthe amplitude modulation is converted to a phase modulation.

Since as described the output signals of the synchronous demodulators 11and 12 also contain information on the amplitude contribution of theindividual signal to the amplitude of the summation signal thisinformation can also be used for a great variety of control andswitching operations. It is advantageous for example to deactivate aspecific individual antenna by means of said signal when the amplitudequantity of the individual signal received by said antenna makes only asmall contribution to the summation signal. The noise contribution ofthis individual signal to the total signal thus predominates and istherefore only detrimental to the total signal.

The clock sequence with which a switchover is made from one antennaoutput signal or from one linear combination to the next antenna outputsignal or next linear combination of output signals for amplitudemodulation, i.e. the frequency of the clock signal S₁, can be selectedin accordance with the requirements and can be constant. In the exampleof embodiment illustrated the clock sequence is controlled via thetrigger circuit 15 by the useful signal itself so that an optimumstarting instant for the successive modulation of the antenna outputsignals or the linear combination of antenna output signals is obtainedtogether with corresponding activation of the phase-shift element forthe corresponding high-frequency individual signal.

The receiving antenna system may also be characterized in that with eachantenna output signal or each linear combination of antenna outputsignals in each case a separate auxiliary modulation signal generatoreach having a different frequency and in each case separate synchronousdemodulators are associated.

The present invention has been described with reference to a preferredexample of embodiment with four antennas. The method and system arehowever suitable for more or less than four antennas. It is for examplealso advantageous to supply to the phase-shift element 4 the clocksignal S_(T) via a delay member so that the phase-shift element 4 is notswitched until the measurement is concluded.

Advantageously, the signal of the auxiliary modulation signal generator16 is supplied to the synchronous demodulators 11 and 12 via a delaymember. This makes it possible to compensate delays due to the receivingcircuit 7, the amplifier and filter circuit 8 and the demodulators 9 to12. Referring to FIG. 3, the auxiliary modulation 16 may beconsecutively superimposed on the individual antenna output signals in apredetermined clock sequence by means of switch 20 connecting modulation16 sequentially to each respective modulation input of modulator 3corresponding to each antenna output signal. Furthermore, modulation 16may also be connected to post summation modulator 18 for use incalibrating the system of the present invention.

We claim:
 1. A method for diversity reception of radio signals using atleast two receiving antennas where the output signal of each receivingantenna is modulated by an auxiliary modulation signal, phase shifted,combined in a summation circuit, amplified and selected in a radioreceiver, coherently demodulated, then used to control the amount ofphase shift made to each antenna output signal such that the resultantsignal summation is maximized for best reception, comprising the stepsof:modulating each of a plurality of antenna output signals with anauxiliary modulation signal; phase-shifting each of said modulatedantenna output signals so that the phase of each of said output signalsare approximately the same; summing each of said modulated andphase-shifted antenna output signals together, wherein a summationsignal is thereby generated; receiving said summation signal byamplifying and selecting said summation signal in a receiver circuit;filtering said received summation signal; demodulating said filteredreceived summation signal in amplitude and frequency demodulators;coherently detecting the demodulated signals from said amplitude andfrequency demodulators in synchronous demodulators, wherein the outputof said amplitude demodulator is coherently detected with said auxiliarymodulation signal and the output of said frequency demodulator iscoherently detected with said auxiliary modulation signal phase shiftedby ninety degrees, whereby the amplitude and phase relationship of eachof said antenna output signals is determined and therefrom the phaseposition and amplitude contribution of each of said antenna outputsignals in relation to said summation signal is derived; and adjustingthe phase-shift of each of said antenna output signals by means of theinformation derived from said synchronous demodulators so that saidsummation signal is optimized for maximum resulting amplitude and bestreception.
 2. Receiving method according to claim 1, characterized inthat the phase of each of said antenna output signals is changed independence upon the phase position determined in predetermined phasestates.
 3. Receiving method according to claim 2, characterized in thatthe phase of each of said antenna output signals is changed depending onthe phase position determined in such a manner that only unilateraldirectional antenna patterns are generated.
 4. Receiving methodaccording to claim 1, characterized in that the phase of each of saidmodulated antenna output signals is rotated in the direction of thephase of the summation signal.
 5. Receiving method according to claim 4,characterized in that the individual antenna patterns are at leastapproximately circular patterns.
 6. Receiving method according to claim1, characterized in that the auxiliary modulation is consecutivelysuperimposed on the individual antenna output signals in a predeterminedclock sequence.
 7. Receiving method according to claim 6, characterizedin that the clock sequence of the clock signal is of the order ofmagnitude of milliseconds.
 8. Receiving method according to claim 6,characterized in that the clock sequence of the clock signal depends onthe speed of a vehicle.
 9. Receiving method according to claim 8,characterized in that the signal for controlling the clock sequence ofthe clock signal is derived from vehicle tachometer.
 10. Receivingmethod according to claim 6, characterized in that the clock sequence ofthe clock signal is controlled by the useful signal.
 11. Receivingmethod according to claim 10, characterized in that the clock sequenceof the clock signal is triggered on occurrence of the zero passage ofthe useful signal.
 12. Receiving method according to claim 1,characterized in that from the antenna output signals linearcombinations are formed on which the auxiliary modulation issuperimposed.
 13. Receiving method according to claim 12, characterizedin that an approximately omnidirectional antenna pattern is formed bylinear combination of said antenna output signals
 14. Receiving methodaccording to claim 1, characterized in that said auxiliary modulationsignal frequencies occur in an unused frequency range of a transmissionchannel.
 15. Receiving method according to claim 1, characterized inthat the antenna output signals contain frequency-modulated stereomultiplex signals and that the antenna output signals modulated with theauziliary modulation contain the auxiliary modulation in a frequencyrange about 57 kHz and/or around 17 and/or 21 kHz of the stereomultiplex signal.
 16. Receiving method according to claim 1,characterized in that the phase difference between the respectiveindividual signal and the summation signal and/or the amplitudecontribution of an individual signal to the amplitude of the summationsignal is converted to a digital signal.
 17. Receiving method accordingto claim 16, characterized in that the phase difference is quantizedwith 2 or 3 bits.
 18. Receiving method according to claim 1,characterized in that an antenna output signal is deactivated when theamplitude contribution from the antenna output signal to the summationsignal drops below a predetermined threshold value.
 19. Receiving methodaccording to claim 1, characterized in that the auxiliary modulation issuperimposed on the respective antenna output signals during a definedtime interval for each of said antenna output signals.
 20. Receivingmethod according to claim 19, characterized in that said antenna outputsignals are video signals and that the auxiliary modulation signalmodulates each of said antenna output signals during the respective lineor frame blanking interval of each of said antenna output signals. 21.Receiving method according to claim 1, further comprising the step ofactivating an antenna output signal when said signal is above apredetermined interference threshold or deactivating an antenna outputsignal when said signal is below a predetermined interference threshold.22. Receiving method according to claim 1, further comprising the stepof modulating said summation signal with a further predetermined definedauxiliary modulation, said modulation of said summation signal is usedas a calibration signal for adjusting the calibration of receivercircuits.
 23. Receiving method according to claim 22, characterized inthat the further auxiliary modulation is identical to the auxiliarymodulation which is superimposed on the individual antenna outputsignals.
 24. Receiving method according to claim 22, characterized inthat the further auxiliary modulation used for the calibration issuperimposed on a summation signal which is formed from a subset of theantenna output signals and which comes closest to the summation signalresulting after the change of the phase and/or amplitude of theindividual signal.
 25. Receiving method according to claim 1,characterized in that an antenna output signal is modulated with amodified auxiliary modulation signal, wherein said antenna output signalappears to be shifted in phase an amount determined by said modifiedauxiliary modulation signal.
 26. Receiving method according to claim 1,further comprising the step of modulating said summation signal bysimultaneous modulation of all the antenna output signals, wherein saidmodulation of said summation signal is used as a calibration signal foradjusting the calibration of receiver circuits.
 27. Apparatus fordiversity reception of radio signals using a plurality of receivingantennas where the signal from each receiving antenna is modulated by anauxiliary modulation signal, phase shifted, combined in a summationcircuit, amplified and selected in a radio receiver, coherentlydemodulated, then used to control the amount of phase shift made to eachantenna output signal such that the resultant signal summation ismaximized for best reception, comprising:a plurality of receivingantennas (1-1, 1-2, 1-3, 1-4); a modulator (3) connected to saidreceiving antennas (1-1, 1-2, 1-3, 1-4) and modulating each of thesignals from said receiving antennas with an auxiliary modulation signal(S_(H)), wherein each of the modulated signals has an auxiliarymodulation component; a phase setting member (4) connected to saidmodulator (3) and adapted to phase-shift each of the modulated signalsfrom said modulator (3); a summation circuit (5) connected to said phasesetting member (4) and adapted to sum the modulated and phase-shiftedsignals from said phase setting member (4) into a summation signal,wherein the summation signal is comprised of the phase shifted antennasignals and auxiliary modulation components thereto; a receiving circuit(7) connected to said summation circuit (5) and adapted to amplify andselect the summation signal therefrom; a filter (8) connected to thereceiving circuit (7) for filtering the auxiliary modulation componentsfrom the summation signal; first and second demodulators (9, 10)connected to said filter (8) and which demodulate auxiliary modulationcomponents in the summation signal; first and second synchronousdemodulators (11, 12) connected to said first and second demodulators,respectively, said synchronous demodulators (11, 12) determining fromthe auxiliary modulation components, the real and imaginary parts ofeach of the signals from said receiving antennas (1-1, 1-2, 1-3, 1-4) inrelation to the summation signal and derives therefrom the phaseposition and amplitude contribution of each of the antenna signals tothe summation signal; and a phase controller connected to saidsynchronous demodulators (11, 12) and converting the phase position andamplitude contribution for each antenna signal into control signals forcontrolling said phase setting member (4), whereby the individualantenna signals are phase-shifted so as to optimize the summation signalfor maximum amplitude and best reception.
 28. Receiving antenna systemaccording to claim 27, characterized in that said phase controller is ananalog/digital converter.
 29. Receiving antenna system according toclaim 27, characterized in that the modulator (3) is activatedconsecutively by means of a clock generator (14).
 30. Receiving antennasystem according to claim 27, characterized in that with each antennaoutput signal a separate auxiliary modulation signal generator eachhaving a different frequency and in each case separate synchronousdemodulators are associated, whereby each separate synchronousdemodulator determines the real and imaginary parts of the respectiveantenna output signal associated with the respective auziliarymodulation signal.
 31. Receiving antenna system according to claim 27,further comprising a matrix circuit (2) connected between the individualreceiving antennas (1-2, 1-2, 1-3, 1-4) and said modulator (3), saidmatrix circuit (2) forms linear combinations of the antenna outputsignals resulting in more uniform antenna reception pattern or improvedsummation of the antenna output signals.
 32. Receiving antenna systemaccording to claim 27 wherein said first and second demodulators (9, 10)consist of amplitude and frequency demodulators, respectively. 33.Receiving antenna system according to claim 32, characterized in thatthe amplitude demodulator (9, 11) is a quasi synchronous demodulator.34. Receiving antenna system according to claim 27 further comprising apost summation modulator (18) connected between the summation circuit(5) and the receiving circuit (7) and connected to auxiliary modulation(16) for modulating the summation signal with the auxiliary modulationsignal (S_(H)).